Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air may be received at a compressor inlet location. Hydrocarbons ingested from purge air, positive crankcase ventilation (PCV) and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, and the accuracy of the sensor, and thus, measurement and/or control of EGR, may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: while EGR is flowing, modulating a reference voltage of an intake manifold oxygen sensor; and estimating an amount of purge and crankcase ventilation hydrocarbons in the EGR based on sensor output during the modulating. In this way, an EGR estimate provided by the intake oxygen sensor can be corrected for the purge and/or PCV content.
For example, during EGR conditions when purging and/or positive crankcase ventilation (PCV) is enabled, the purge and/or blow-by gas hydrocarbons may react with oxygen at the sensor to generate water and carbon dioxide. Therefore, when EGR is flowing and purge and PCV are enabled, a reference voltage applied to the intake manifold oxygen sensor may be alternated between a higher reference voltage and a lower reference voltage. The lower voltage may be a nominal voltage, such as 450 mV, which does not allow for dissociation of water or carbon dioxide molecules, while the higher voltage may be at or above a threshold voltage, such as at or above 800 mV, which does allow for disassociation of the products of the reacting hydrocarbons (that is, water and carbon dioxide). By comparing a pumping current output by the sensor at the higher and lower voltages, the change in oxygen concentration can be used to infer a total amount of purge and PCV hydrocarbons contained in the air charge Then, purge may be selectively disabled, for example by closing a purge valve. The reference voltage may then be modulated again between the higher and lower reference voltages. A difference between the pumping currents output by the sensor at the higher and lower reference voltages, in the absence of purge air, may be used to infer a PCV content of the air charge. The total amount of purge and PCV hydrocarbons in the air charge (estimated with purge enabled) and the PCV content of the air charge (estimated with purge disabled) may then be used to compute the purge content of the air charge. The controller may then correct an EGR estimate based on the learned purge and PCV content, and use the corrected EGR estimate for more accurate EGR flow control.
In this way, by applying a higher reference voltage to an intake manifold oxygen sensor during purging and crankcase ventilation conditions, the effect of the ingested hydrocarbons on the output of the sensor can be nullified. In addition, by comparing the output of the sensor at the higher reference voltage to a sensor output at a lower, nominal reference voltage, each of a purge and PCV content of the air charge may be determined. By correcting the EGR estimated by the intake manifold oxygen sensor for the purge and PCV hydrocarbon content, the corruption of the sensor output by purge air or blow-by gas hydrocarbons is nullified. By improving the accuracy of EGR dilution estimation in the presence of purge air or crankcase gases, EGR control can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.